[Cs3] consts with maps

[Mikhail Nesterenko]

CS3,

Here is the story. In a map, key is always implicitly const. Its modification is
not allowed. 

    map<int, double> mymap; // implies that the integer key is not modifiable

An attempt to change a key through iterator should fail as the
iterators const-ify reference to the key.

However,

    map<const int, double>  mymap; // is allowed, is somewhat redundant
                                   // and is treated as a separate type
				   // the above map

Now, as far as value, it can be declared const 

    map<int, const double>  mymap;

In which case, the value cannot be modified. Also, the map itself can
be declared const:

    const map<int, double>  mymap = {{0, 0.}, {1, 1.}};

In which case, the map cannot be modified altogether. There seems to
be a lot of frustrating on the internet since indexing operator
(that can potentially modify the map) is not allowed for constant
maps.

Thanks,
-- 
Mikhail